Intensity modulated radiation therapy (IMRT) uses the capability of modern linear accelerators to adjust (modulate) the radiation intensity delivered from multiple ports, greatly increasing control over the delivered dose distribution. The ports are delivered in a time sequence, which for an immobile patient will sum to the desired dose. While successful in many sites, one reason IMRT has not yet been used for lung cancer is because the lung and tumor move significantly during breathing, causing significant dose distribution delivery errors. Breathing motion can be reduced using breath-hold techniques, but many lung cancer patients are unable to hold their breath for the required length of time. The overall hypothesis of this grant is that the local control of lung cancer can be significantly improved, at fixed or reduced morbidity levels, by advances in three-dimensional (3D) imaging, treatment planning, and delivery which overcome dose delivery errors due to breathing motion. Specific aim 1 will determine that a spirometer, a device that quantitatively measures the volume of air moving in and out of the lungs, can be used as an independent quantity against which the internal lung motion is mapped. Specific aim 2 will use the quantitative spirometry, multislice CT scanning, and sophisticated deformable image analysis to develop a 3D model of breathing motion. Specific aim 3 will develop a dose calculation model that models the dose distribution to a breathing patient, showing the dose distribution variation (errors) due to breathing motion. Gating the linear accelerator to a portion of the breathing cycle has the potential for reducing the dose distribution variations. The model utility will be demonstrated by calculating the influence of linear accelerator gating on free-breathing IMRT patients, determining the accuracy of dose delivery as a function of treatment delivery efficiency.